1. Field of the Invention
The present invention relates generally to semiconductor devices having rings disposed about the peripheries of the contact pads thereof and, more specifically, to the use of stereolithography to fabricate such rings around the contact pads either before or after securing solder balls to the contact pads. Particularly, the present invention pertains to rings disposed about the peripheries of the contact pads of a semiconductor device component for enhancing the reliability of solder balls secured to the contact pads. The present invention also relates to semiconductor device components including such rings.
2. State of the Art
Reliability of Solder Balls Used to Connect a Semiconductor Device Face-Down to a Higher Level Substrate
Some types of semiconductor devices, such as flip-chip type semiconductor dice, including ball grid array (BGA) packages, and chip-scale packages (CSPs), can be connected to higher level substrates by orienting these semiconductor devices face down over the higher level substrate. The contact pads of such semiconductor devices are typically connected directly to corresponding contact pads of the higher level substrate by solder balls.
Examples of solders that are known in the art to be useful in connecting semiconductor devices face down to higher level substrates include, but are not limited to, lead-tin (Pb/Sn) solder and silver-nickel (Ag/Ni) solder. For example, 63/37 type Pb/Sn solder bumps (i.e., solder having about 63% by weight lead and about 37% by weight tin) and 95/5 type Pb/Sn solder bumps (i.e., solder having about 95% by weight lead and about 5% by weight tin) have been used in flip-chip, ball grid array, and chip-scale packaging type attachments.
Assemblies that include semiconductor devices connected face down to higher level substrates using solder balls are subjected to thermal cycling during subsequent processing, testing thereof, and in normal use. As these assemblies undergo thermal cycling, the solder balls thereof are also exposed to wide ranges of temperatures, causing the solder balls to expand when heated and contract when cooled. Such expansion and contraction is especially problematic at the interface between a solder ball and the underlying contact pad. Expansion and contraction of solder balls can also occur at the interface between a solder ball and the contact pad of a higher level substrate to which, for example, a die is secured. Repeated variations in temperatures can cause solder fatigue, which can reduce the strength of the solder balls, cause the solder balls to crack and fail, and diminish the reliability of the solder balls as mechanical and electrical connection elements.
In an attempt to increase the reliability with which solder balls connect semiconductor devices face down to higher level substrates, resins have been applied to semiconductor devices to form rings around the bases of the solder balls protruding from the semiconductor devices. These resinous supports laterally contact the bases of the solder balls to enhance the reliability thereof. The resinous supports are applied to a semiconductor device after solder balls have been secured to the contact pads of the semiconductor device and before the semiconductor device is connected face down to a higher level substrate. As those of skill in the art are aware, however, the shapes of solder balls can change when bonded to the contact pads of a substrate, particularly after reflow thereof. If the shapes of the solder balls change, the solder balls can fail to maintain contact with the resinous supports, which could thereby fail to protect or enhance the reliability of the solder balls.
The inventor is not aware of any art that discloses a method that can be used to fabricate support rings around the contact pads of a semiconductor device before, as well as after, solder balls are secured to the contact pads.
Stereolithography
In the past decade, a manufacturing technique termed xe2x80x9cstereolithographyxe2x80x9d, also known as xe2x80x9clayered manufacturingxe2x80x9d, has evolved to a degree where it is employed in many industries.
Essentially, stereolithography, as conventionally practiced, involves utilizing a computer to generate a three-dimensional (3-D) mathematical simulation or model of an object to be fabricated, such generation usually effected with 3-D computer-aided design (CAD) software. The model or simulation is mathematically separated or xe2x80x9cslicedxe2x80x9d into a large number of relatively thin, parallel, usually vertically superimposed layers, each layer having defined boundaries and other features associated with the model (and thus the actual object to be fabricated) at the level of that layer within the exterior boundaries of the object. A complete assembly or stack of all of the layers defines the entire object and surface resolution of the object is, in part, dependent upon the thickness of the layers.
The mathematical simulation or model is then employed to generate an actual object by building the object, layer by superimposed layer. A wide variety of approaches to stereolithography by different companies has resulted in techniques for fabrication of objects from both metallic and nonmetallic materials. Regardless of the material employed to fabricate an object, stereolithographic techniques usually involve disposition of a layer of unconsolidated or unfixed material corresponding to each layer within the object boundaries. This is followed by selective consolidation or fixation of the material to at least a partially consolidated, or semisolid, state in those areas of a given layer corresponding to portions of the object, the consolidated or fixed material also at that time being substantially concurrently bonded to a lower layer of the object to be fabricated. The unconsolidated material employed to build an object may be supplied in particulate or liquid form and the material itself may be consolidated or fixed or a separate binder material may be employed to bond material particles to one another and to those of a previously formed layer. In some instances, thin sheets of material may be superimposed to build an object, each sheet being fixed to a next lower sheet and unwanted portions of each sheet removed, a stack of such sheets defining the completed object. When particulate materials are employed, resolution of object surfaces is highly dependent upon particle size. When a liquid is employed, surface resolution is highly dependent upon the minimum surface area of the liquid which can be fixed and the minimum thickness of a layer that can be generated. Of course, in either case, resolution and accuracy of object reproduction from the CAD file is also dependent upon the ability of the apparatus used to fix the material to precisely track the mathematical instructions indicating solid areas and boundaries for each layer of material. Toward that end, and depending upon the layer being fixed, various fixation approaches have been employed, including particle bombardment (electron beams), disposing a binder or other fixative (such as by ink-jet printing techniques), or irradiation using heat or specific wavelength ranges.
An early application of stereolithography was to enable rapid fabrication of molds and prototypes of objects from CAD files. Thus, either male or female forms on which mold material might be disposed might be rapidly generated. Prototypes of objects might be built to verify the accuracy of the CAD file defining the object and to detect any design deficiencies and possible fabrication problems before a design was committed to large-scale production.
In more recent years, stereolithography has been employed to develop and refine object designs in relatively inexpensive materials and has also been used to fabricate small quantities of objects where the cost of conventional fabrication techniques is prohibitive for the same, such as in the case of plastic objects conventionally formed by injection molding. It is also known to employ stereolithography in the custom fabrication of products generally built in small quantities or where a product design is rendered only once. Finally, it has been appreciated in some industries that stereolithography provides a capability to fabricate products, such as those including closed interior chambers or convoluted passageways, which cannot be fabricated satisfactorily using conventional manufacturing techniques. It has also been recognized in some industries that a stereolithographic object or component may be formed or built around another, pre-existing object or component to create a larger product.
However, to the inventor""s knowledge, stereolithography has yet to be applied to mass production of articles in volumes of thousands or millions, or employed to produce, augment or enhance products including other, pre-existing components in large quantities, where minute component sizes are involved, and where extremely high resolution and a high degree of reproducibility of results is required. In particular, the inventor is not aware of the use of stereolithography to fabricate peripheral rings around the contact pads of semiconductor devices, such as flip-chip type semiconductor devices or ball grid array packages. Furthermore, conventional stereolithography apparatus and methods fail to address the difficulties of precisely locating and orienting a number of pre-existing components for stereolithographic application of material thereto without the use of mechanical alignment techniques or to otherwise assuring precise, repeatable placement of components.
The present invention includes a dielectric ring that surrounds the periphery of a contact pad of a semiconductor device, semiconductor device components including such rings, and methods for fabricating the rings.
A ring incorporating teachings of the present invention surrounds the periphery of a contact pad exposed at the surface of a semiconductor device component, such as a semiconductor die, a chip-scale package substrate, or a carrier substrate. The ring protrudes from the surface of the semiconductor device component. If the ring is fabricated before a solder ball is secured to the contact pad, at least a portion of the surrounded contact pad is exposed through an aperture defined by the ring.
Since the ring protrudes from the surface of the semiconductor device component, when a solder ball is bonded or otherwise secured to the contact pad exposed through the ring, the ring laterally surrounds at least a portion of the solder ball. The ring is preferably configured to substantially conformably contact a solder ball surrounded thereby so as to laterally support at least the contacted portion of the solder ball. Such conformance is enhanced during reflow of the solder ball, where any substantial voids between the solder and the interior of the ring are eliminated. Accordingly, during thermal cycling of the semiconductor device, the ring accommodates expansion and contraction of the solder ball and, most particularly, the portion thereof that contacts and is metallurgically secured to the underlying contact pad, the ring thereby reducing the occurrence of solder fatigue. In addition, use of rings according to the present invention, which may be of substantial height or protrusion from a substrate so as to encompass the solder balls at or approaching their largest diameters, may eliminate the need for an insulative underfill conventionally applied between a die and a higher level substrate.
Another significant advantage of the rings of the present invention is the containment of the solder of the balls, in the manner of a dam, during solder reflow, thus preventing contamination of the passivation layer surrounding the contact pads.
According to another aspect, the present invention includes a method for fabricating the ring. In a preferred embodiment of the method, a computer-controlled, 3-D CAD-initiated process known as xe2x80x9cstereolithographyxe2x80x9d or xe2x80x9clayered manufacturingxe2x80x9d is used to fabricate the ring. When stereolithographic processes are employed, each ring is formed as either a single layer or a series of superimposed, contiguous, mutually adhered layers of material.
The stereolithographic method of fabricating the rings of the present invention preferably includes the use of a machine vision system to locate the semiconductor devices or other substrates on which the rings are to be fabricated, as well as the features or other components on or associated with the semiconductor devices or other substrates (e.g., solder bumps, contact pads, conductor traces, etc.). The use of a machine vision system directs the alignment of a stereolithography system with each semiconductor device or other substrate for material disposition purposes. Accordingly, the semiconductor devices or other substrates need not be precisely mechanically aligned with any component of the stereolithography system to practice the stereolithographic embodiment of the method of the present invention.
In a preferred embodiment, the rings to be fabricated upon or positioned upon and secured to a semiconductor device component in accordance with the invention are fabricated using precisely focused electromagnetic radiation in the form of an ultraviolet (UV) wavelength laser under control of a computer and responsive to input from a machine vision system, such as a pattern recognition system, to fix or cure selected regions of a layer of a liquid photopolymer material disposed on the semiconductor device or other substrate.
The rings of the present invention may be fabricated around the contact pads of the semiconductor device component either before or after solder balls are bonded or otherwise secured to the contact pads.
Other features and advantages of the present invention will become apparent to those of skill in the art through consideration of the ensuing description, the accompanying drawings, and the appended claims.